This application claims the priority of Japanese Patent Application No. 4-348884 filed Dec. 28, 1992, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a secondary battery and, in particular, to improvement in the binder used therefor to improve its cycle characteristics.
2. Description of the Prior Art
Carbon materials such as coke and graphite have been proposed in recent years as negative electrode materials for secondary batteries with organic electrolyte solutions, for the benefits of good flexibility, no fear of depositing of mossy lithium and the like.
Negative electrodes utilizing the above carbon materials are generally prepared by a process which comprises the successive steps of dispersing a carbon powder (e.g. graphite and coke powder) and, as required, a conductive agent powder (e.g. acetylene black and carbon black) in a binder solution to obtain a slurry, applying the slurry to a collector metal using doctor blade, or by like methods, and drying the slurry on the metal.
Japanese Patent Publication No. 23433/1987 proposes a rechargeable lithium battery utilizing a negative electrode comprising as its negative electrode active material a graphite intercalation compound incorporating lithium, in which a fluororesin is used as a binder.
However, with graphite or the like incorporating lithium, the use of fluororesin as a binder causes, upon repeated charges and discharges, the carbon powder to peel off from the substrate (copper plate, copper foil or the like) of the negative electrode due to insufficient strengths for bonding the active material itself and for bonding the active material with the substrate. In this case, as a result, the battery capacity gradually decreases.
Besides, generally secondary batteries utilizing fluororesin have the problem of short cycle life. Furthermore, when the battery temperature rises due to short circuit or like accidents, the fluororesin decomposes to form HF (hydrogen fluoride), which then reacts (exothermic reaction) vigorously with C.sub.6 Li that has formed in the negative electrode by charge, whereby the battery may break or burst. This also constitutes a problem with respect to reliability (safety) of such batteries.